Why Does Using `% v` in a `String()` Method Lead to Infinite Recursion with Nested Structs?

Infinite Recursive Stack Overflow with Nested Structs in String() Function

Consider the following nested structs:

type ConfigOne struct {
    Daemon daemon
}

type daemon struct {
    Loglevel int
    Logfile string
}

Now, let's assume we define a String() string method on the ConfigOne type:

func (c ConfigOne)String() string {
    return fmt.Sprintf("%+v\n", c)
}

This String() method attempts to return the values of the nested struct elements by using the % v format specifier. However, this leads to an infinite recursive stack overflow error.

The reason for this error is that the %v and % v format specifiers use the value of the String() function if the type implements it. Therefore, using % v on a type within the String() function for that type creates a recursive call that never ends.

To avoid this error, you should not use % v in your String() function. Instead, you should manually construct your string by explicitly accessing and formatting the field values:

func (c ConfigOne)String() string {
    return fmt.Sprintf("Loglevel: %d, Logfile: %s\n", c.Daemon.Loglevel, c.Daemon.Logfile)
}

This modified String() function no longer causes an infinite recursion, and it correctly outputs the values of the nested struct elements.

Alternatively, you can override the String() method in the nested daemon struct to prevent the recursion:

func (d daemon)String() string {
    return fmt.Sprintf("Loglevel: %d, Logfile: %s\n", d.Loglevel, d.Logfile)
}

By overriding the String() method in the nested struct, you break the recursive call chain and ensure that the String() function only outputs the values of the nested struct itself, rather than all its containing structures.

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